Structure and method of making double-gated self-aligned finfet having gates of different lengths

ABSTRACT

A gated semiconductor device is provided, in which the body has a first dimension extending in a lateral direction parallel to a major surface of a substrate, and second dimension extending in a direction at least substantially vertical and at least substantially perpendicular to the major surface, the body having a first side and a second side opposite the first side. The gated semiconductor device includes a first gate overlying the first side, and having a first gate length in the lateral direction. The gated semiconductor device further includes a second gate overlying the second side, the second gate having a second gate length in the lateral direction which is different from, and preferably shorter than the first gate length. In one embodiment, the first gate and the second gate being electrically isolated from each other. In another embodiment the first gate consists essentially of polycrystalline silicon germanium and the second gate consists essentially of polysilicon.

BACKGROUND OF INVENTION

The present invention relates to semiconductor devices and theirmanufacture, and more specifically to a structure and method of making afield effect transistor (FET) of the finFET type having dual gates.

Field effect transistors (FETs) can be formed in a variety of ways toserve a variety of purposes for integrated circuits and other devices.Commonly, FETs are formed as “planar” devices in many integratedcircuits, i.e., as devices in which the conduction channel has width andlength extending in a direction parallel to the major surface of asubstrate. FETs can be formed in a semiconductor-on-insulator (SOI)layer of a substrate or in a bulk semiconductor substrate. Frequently,FETs are formed having a non-planar conduction channel, in order toserve a special purpose. In such non-planar FETs, either the length orthe width of the transistor channel is oriented in the verticaldirection, that is, in a direction perpendicular to the major surface ofthe substrate. In one such type of device, called the finFET, the widthof the conduction channel is oriented in the vertical direction, whilethe length of the channel is oriented parallel to the major surface ofthe substrate. With such orientation of the channel, finFETs can beconstructed to have a larger width conduction channel than planar FETsso as to produce larger current drive than planar FETs which occupy thesame amount of integrated circuit area (the area parallel to the majorsurface of the substrate). Hence, it is desirable to incorporate finFETsin some types of integrated circuits where large current drive is neededand compactness of the device is important.

However, the fabrication of finFETs poses challenges. To improvemobility in the channel and enhance performance, finFETs have lowerdoping levels than typical semiconductor-on-insulator (SOI) devices, andhave lower doping levels than typical devices formed in bulk substrates(hereinafter, “bulk devices”). This makes the threshold voltage of thefinFET difficult to control. In some finFETs, this problem is addressedby providing two independently controlled gates for the finFET, oneprimary gate which controls the normal switching and/or amplificationfunction of the transistor, and another gate known as a “back gate,” or“weak gate” which is used to adjust the threshold voltage of the finFET.Unfortunately, the additional gate is not without cost to the operationof the finFET. The back gate increases the capacitance of the finFET.Ultimately, the increase capacitance negatively affects the maximumswitching speed and/or peak operating frequency of the transistor.

Accordingly, there is a need to provide an improved structure and methodof making a dual gate finFET having a back gate for adjusting thethreshold voltage of the finFET, while adding less capacitance to thedevice.

SUMMARY OF INVENTION

According to an aspect of the invention, a gated semiconductor device isprovided, having a fin-shaped body which has a first dimension extendingin a lateral direction parallel to a major surface of a substrate, and asecond dimension extending in a direction at least substantiallyvertical and at least substantially perpendicular to the major surface,the body having a first side and a second side opposite the first side.The gated semiconductor device includes a first gate overlying the firstside, and having a first gate length in the lateral direction. The gatedsemiconductor device further includes a second gate overlying the secondside, the second gate having a second gate length in the lateraldirection which is different from, and preferably shorter than the firstgate length, the first gate being electrically isolated from the secondgate.

According to another aspect of the invention, a gated semiconductordevice is provided having a fin-shaped body which has a first dimensionextending in a lateral direction parallel to a major surface of asubstrate, and a second dimension extending in a direction at leastsubstantially vertical and at least substantially perpendicular to themajor surface, the body having a first side and a second side oppositethe first side. The gated semiconductor device includes a first gateconsisting essentially of polycrystalline silicon germanium overlyingthe first side, and having a first gate length in the lateral direction.The gated semiconductor device further includes a second gate consistingessentially of polysilicon overlying the second side, the second gatehaving a second gate length in the lateral direction which is differentfrom, and preferably shorter than the first gate length.

According to another aspect of the invention, a method is provided ofmaking a gated semiconductor device. Such method includes patterning asingle-crystal semiconductor region of a substrate to extend in alateral direction parallel to a major surface of a substrate and toextend in a direction at least substantially vertical and at leastsubstantially perpendicular to the major surface, the semiconductorregion having a first side and a second side opposite the first side. Afirst gate consisting essentially of polycrystalline silicon germaniumis formed overlying the first side, the first gate having a first gatelength in the lateral direction. A second gate consisting essentially ofpolysilicon is formed overlying the second side, the second gate havinga second gate length in the lateral direction which is different from,and preferably shorter than the first gate length.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are a plan view, and cross-sectional view, respectively,illustrating the structure a gated semiconductor device, such as afinFET, according to an embodiment of the invention.

FIG. 3 is a graph illustrating relative reduction in capacitance of aweak gate of a double-gated semiconductor device, relative to a stronggate, for various relative reductions in the gate length of the weakgate.

FIGS. 4 through 16 are cross-sectional views illustrating stages in thefabrication of a gated semiconductor device according to an embodimentof the invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 are a plan view and a corresponding cross-sectional view,respectively, illustrating the structure of a gated semiconductor device10 according to an embodiment of the invention. In one arrangement foruse in an integrated circuit, the gated semiconductor device is operableas an insulated gate field effect transistor (FET) having a fin-shapedbody 12, a source 14 disposed at one end of the body, and a drain 16disposed at the other end of the body 12. In such arrangement, the gatedsemiconductor device 10 can be referred to as a finFET. Alternatively,in another arrangement in which one of the source and the drain isconductively connected to the gate of the transistor, the gatedsemiconductor device is operable as a gated diode device. With specificreference to FIG. 2, the body 12 is preferably disposed in a SOI layeroverlying a buried oxide region 15 of an SOI substrate, the buried oxideregion having an upper surface 26. The body 12 of the device 10 extendsin a direction 22 (FIG. 1) which is parallel to the upper surface 26 ofthe BOX layer. The upper surface 26 of the BOX layer lies in a planewhich is generally parallel to the major surface of the substrate. Thebody 12 of the device 10, which extends in a direction parallel to theupper surface of the BOX layer, thus extends in the lateral direction 22which is parallel to the major surface of the substrate. The body 12also extends in a direction 25 which is at least substantially verticaland at least substantially perpendicular to the upper surface of the BOXlayer 26, i.e., in a direction at least substantially perpendicular tothe major surface of the substrate. The body 12 further has a first side28 over which a first gate 18 is disposed and a second side 30 overwhich a second gate 20 is disposed. The first gate 18 has a first gatelength 60 (FIG. 1) which is substantially different from the second gatelength 62 (FIG. 1) of the second gate 20. In a preferred embodiment, thefirst gate length 60 is substantially shorter than the second gatelength 62, i.e., by 10% or more of the second gate length 62, and morepreferably by 20% to 60% of the second gate length. In a preferredembodiment, the first gate length and the second gate length are madevery small. For example, the second gate length is nominally set to 25nm, and the first gate length is set to 20 nm. The first gate 18 isinsulated from the body 12 by a first gate dielectric 29 and the secondgate 20 is insulated from the body 12 by a second gate dielectric 31. Incommon usage, when a double-gated semiconductor device is electricallyconnected for use as a transistor, one of the gates is used primarilyfor operating the transistor, and can be referred to as a “top gate” or“strong gate”, while the other of the gates is used primarily forbiasing the transistor to adjust the threshold voltage VT of thetransistor and is referred to as a “bottom gate” or “weak gate”. Withrespect to the gated semiconductor device 10 of the present embodiment,when the thicknesses of the first gate dielectric 29 and the second gatedielectric 31 are the same, the second gate 20 functions as the “stronggate” and the first gate 18 functions as the “weak gate”.

An advantage of providing a weak gate having a shorter gate length thanthe strong gate of the device 10 is to reduce the gate capacitance dueto the weak gate, thus reducing the impact of the weak gate on theperformance of the device 10. Here, the gate length of the first gate(the weak gate) is reduced relative to the gate length of the secondgate (the strong gate) to reduce unnecessary capacitance. FIG. 3 is agraph illustrating relative reductions in gate capacitance, obtainedthrough computer simulation, for devices having different first gatelengths, which are shortened by different degrees relative to the secondgate length. The data illustrated in FIG. 3 is for a device in which thelength of the second gate (the strong gate) is 25 nm. As shown in FIG.3, the left most “bar” 70 indicates the reduction in capacitance of thefirst gate (the weak gate) in relation to the second gate (the stronggate) when the first gate has a gate length of 25 nm. Since for that bar70 the first gate length (25 nm) is the same as the second gate length(25 nm), there is no reduction in the gate capacitance of the firstgate. As further shown at 72 in FIG. 3, when the first gate length isreduced to 21 nm, the gate capacitance due to the first gate is reducedby 13% relative to the gate capacitance due to the second gate. When thefirst gate length is reduced to 18 nm, as shown at 74, a still greater(21%) reduction in the gate capacitance due to the first gate isachieved. When the length of the first gate is reduced to 16 nm, asshown at 76, a 26% reduction in the gate capacitance due to the firstgate is achieved.

Referring again to FIG. 2, the body has a top surface 24 over which aninsulating cap 32 is disposed. As further shown in FIG. 1, the gatedsemiconductor device 10 also includes oxide regions 40 disposed alongedges of the first gate 18, the oxide regions 40 having a compositionincluding an oxide of silicon and an oxide of germanium. Dielectricspacers 43 are disposed laterally adjacent to the oxide regions 40, thedielectric spacers 43 preferably consisting essentially of a nitride,e.g., silicon nitride. A first polysilicon gate layer 42 is alsodisposed overlying the first gate, the first polysilicon gate layerpreferably at least partly overlying each of the oxide regions 40disposed to either side of the first gate 18. In addition, oxide regions41 are disposed along edges of the second gate, the oxide regions 41consisting essentially of an oxide of silicon. In addition, dielectricspacers 45 are disposed laterally adjacent to the oxide regions 41, thedielectric spacers 45 preferably consisting essentially of a nitride,e.g., silicon nitride. With reference to FIG. 2, in one embodiment, asilicide layer 46 is disposed on the upper surface 44 of the firstpolysilicon gate layer 42, the upper surface 50 of the second gate 20,and overlying each of the source and drain. Finally, an interleveldielectric (ILD) 55 is provided over the silicide layer 46. A firstconductive via 52 conductively contacts the silicide layer 46 overlyingthe first gate polysilicon layer, another conductive via 54 conductivelycontacts the silicide layer 46 overlying the second gate 20, and otherconductive vias 56 and 58 conductively contact the silicide layer 46overlying the source 14 (FIG. 1) and the drain 16 (FIG. 1).

A method of making a self-aligned double-gated semiconductor deviceaccording to an embodiment of the invention will now be described withadditional reference to FIGS. 4 through 16. Preferably, the gatedsemiconductor device is fabricated in a single-crystalsemiconductor-on-insulator (“SOI”) layer 102 or a silicon-on-insulatorlayer of an SOI substrate 100 in which the SOI layer 102 is disposedoverlying a buried oxide layer 104. The buried oxide layer 104, in turn,is disposed over a bulk semiconductor region 106 of the substrate. Athin oxide 108 is disposed on the top surface 110 of the SOI layer 102,such as that which is commonly used as a gate dielectric of other FETsbeing manufactured on the SOI layer 102. Here, thin oxide layer 108functions as an etch stop layer in the fabrication of the gatedsemiconductor device 10 (FIGS. 1 and 2). A layer of polycrystallinesilicon germanium (“poly-SiGe”) 112 is thereafter formed over the thinoxide 108, after which a photoresist layer 114 is deposited andpatterned to form an opening 116 over a portion of the SOI layer 102.

Thereafter, as shown in FIGS. 5 and 6, the patterned photoresist layer114 is used as a mask to transfer the pattern to the poly-SiGe layer112, as by a directional reactive ion etch (“RIE”) that is firstperformed selective to the oxide in oxide layer 108, so as to stopetching when the oxide layer 108 is reached. Thereafter, the oxide layer108 is removed as by etching for a short duration, e.g., through anisotropic etch with a reagent such as hydrofluoric acid (“HF”).Subsequent thereto, a sidewall spacer 120 is formed on a sidewall 122 ofthe poly-SiGe layer 112 through a well-known technique such asdepositing a conformal layer of dielectric material, e.g., siliconnitride, over the poly-SiGe layer 112 and other layers, and thenvertically etching the conformal layer, as by RIE, selective to thematerial, e.g., silicon, of the underlying SOI layer 102.

Subsequent thereto, as shown in FIG. 7, a thick layer 124 of oxide isdeposited over the poly-SiGe layer. Thereafter, the oxide layer 124 isplanarized as shown in FIG. 8, by, for example, performing chemicalmechanical polishing (“CMP”) selective to polycrystalline silicongermanium and selective to nitride, stopping on the top of the nitridespacer. This results in the structure as shown in FIG. 8 in which a topsurface 128 of the oxide layer is planarized to a top surface 126 of thepoly-SiGe layer 112.

Thereafter, as shown in FIG. 9, the SOI layer is etched again, stoppingon the BOX layer 104. This etch is preferably performed by RIE to etchthrough the poly-SiGe layer 112 (FIG. 8), thin oxide and SOI layer toproduce a “fin” structure 130 from the poly-SiGe layer which overliesthe BOX layer 104. In processing up to this point, the choice ofpolycrystalline silicon germanium is favored as the material of thelayer 112 when the fin structure 130 is etched because it tends to etchat a faster rate than polysilicon or single-crystal silicon, when allother characteristics of the materials are considered to be equal. Thefaster etching rate of polycrystalline silicon germanium helps to fullyremove that layer 112 when the fin structure 130 is patterned, whileprotecting the fin structure from erosion during that patterning. Theresulting fin structure 130 has dimension 118 in the vertical directionwhich equates at least generally to a width of the conduction channel ofthe gated semiconductor device when completed. The patterning of the SOIlayer is preferably performed by etching the SOI layer selective to theoxide of the underlying BOX layer 104 so as to stop on the BOX layer104, as shown.

Thereafter, as shown in FIG. 10, a first gate dielectric 132 is formedon a sidewall 133 of the fin structure 130, after which a further layerof polycrystalline silicon germanium (“poly-SiGe”) 134 is formed in aconformal deposition process. Illustratively, the thin dielectric 132 isformed by thermal oxidation and/or thermal nitridation and/or depositionof a high dielectric constant (“high-K”) dielectric material.Thereafter, as shown in FIG. 11, a further layer of oxide 138 isdeposited over the structure and then planarized to the top surface 128of the pre-existing oxide layer 124, such as by a further chemicalmechanical polishing (CMP) step.

Thereafter, referring to FIG. 12, the oxide layers 124 and 138 shown inFIG. 11 are removed, as by a process of etching the oxide selective tothe materials that are to remain after the etch, i.e., the poly-SiGelayer 134. Here, RIE or an isotropic etch selective to the poly-SiGelayer 134 is sufficient to remove the oxide layers 124, 138.

FIG. 13 shows a stage of processing subsequent to that shown in FIG. 12.At this stage, a gate dielectric 141 is formed on the second sidewall139 of the fin structure. At the same time, a thin dielectric 142 isformed on the outer surface 140 of the poly-SiGe layer 134.Illustratively, the thin dielectric 142 is formed by thermal oxidationand/or thermal nitridation and/or deposition of a high dielectricconstant (“high-K”) dielectric material onto the top surface 140 of thepoly-SiGe layer 134. Thereafter, relatively thin spacers 144 ofpolysilicon are formed on the vertical sidewalls of the structure. Thepolysilicon spacer 144 protects the gate dielectric 141 covering thesecond sidewall 139 of the fin structure 130 during the subsequentremoval of the thin dielectric 142 from most of the outer surface 140 ofthe poly-SiGe layer 134.

Later, as shown in FIG. 14, polysilicon is deposited conformally overthe structure to form a layer 150, after which an oxide is deposited tocover the essentially horizontal surfaces 154, 156 of the polysiliconlayer 150 through a directional deposition process such as high-densityplasma (HDP) deposition. This results in a relatively thin layer 151 ofoxide covering the uppermost horizontal surface 154 of the polysiliconlayer 150, while forming thicker regions 152 covering the lowerhorizontal surfaces 156 of layer 150.

Thereafter, as shown in FIG. 15, the thin layer 151 of oxide (FIG. 14)is removed to expose the uppermost horizontal surface, after which thepolysilicon layer 150 (FIG. 14) is then etched back, such that thenitride spacer 120 is exposed above the fin structure. These steps arepreferably performed using an HF based oxide chemical etch to removeoxide layer 151, followed by a wet chemical etch of the polysiliconlayer 150, to result in the separation of that layer into a first gatepolysilicon layer 42 and the second gate 20. Alternatively, withreference to FIG. 14, another way to separate the first gate from thesecond gate is to deposit an oxide, and thereafter perform CMP, stoppingon the top of the polysilicon layer 150 above the fin structure, andetching the exposed portion of the polysilicon layer 150 to separate thepolysilicon layer into the first gate polysilicon layer 42 and thesecond gate 20 that are shown in FIG. 15. In still another alternative,the CMP process can be conducted to stop on the nitride spacer 120,followed by etching the exposed polysilicon layer to separate that layerinto the first gate polysilicon 42 and the second gate 20.

Thereafter, the oxide layer covering the first gate polysilicon 42 andthe second gate 20 are removed, as by wet etching, to produce thestructure shown in FIG. 16. Finally, with reference to FIGS. 1 and 2,steps are performed to complete the self-aligned gated semiconductordevice 10. Such steps include the patterning of the first gatepolysilicon layer 42 and the second gate 20 in the lateral direction 22using one photolithographic mask, followed by oxidation of the sidewallsof the first gate 18 to form the spacers 40, and oxidation of thesidewalls of the second gate 20 to form the spacers 41 having a widthdifferent from the widths of spacers 40, as shown in FIG. 1. In thiscase, the first gate, i.e., the poly-SiGe gate has smaller gate lengththan the second gate, consisting essentially of polysilicon, due to thefact that poly-SiGe has a faster oxidation rate than does polysilicon.Accordingly, the oxidation process consumes more SiGe than Si under thesame oxidation conditions. Thereafter, further spacers 43 and 45 areformed on sidewalls of the respective spacers 40 and 41. Ion implantsare now performed to the source 14 and drain 16, as masked by the firstand second gates, spacers 43, 45 and nitride cap 32. Thereafter, asilicide layer 46 is formed, preferably by a self-aligned technique,overlying and self-aligned to individual component portions of the firstgate polysilicon layer 42, second gate 20, source 14 and drain 16, suchthat the silicide layer 46 overlying each component portion is insulatedfrom each other portion. Finally, an interlevel dielectric 55 is formedover the structure and contact vias 52, 54, 56 and 58 are then etchedand filled to conductively contact the silicide layer overlying each ofthe first gate polysilicon layer 42, second gate 20, source 14 and drain16 to complete the gated semiconductor device 10.

While the invention has been described in accordance with certainpreferred embodiments thereof, those skilled in the art will understandthe many modifications and enhancements which can be made theretowithout departing from the true scope and spirit of the invention, whichis limited only by the claims appended below.

For example, in an alternative embodiment, the gated semiconductordevice is fabricated in a single-crystal region of a bulk semiconductorsubstrate (not shown), and is fabricated by the same process asdiscussed above, except as referring to FIG. 7, a timed etch defines thewidth of the conduction channel, instead of the thickness 118 of the SOIlayer defining the width of the conduction channel.

Moreover, referring to FIG. 2, it is not essential for the body 12 ofthe gated semiconductor device 10 to have a top side 24 and aninsulating cap 32 overlying the top side 24. In an alternativeembodiment, the first side 28 of the gated semiconductor device isconnected directly to the second side 30, eliminating the top side 24.In such case, isolation between the first gate and the second isachieved by vertically displacing the first gate relative to the secondgate and isolating the gates from each other by a dielectric isolationregion covering the body 12 between the first gate and the second gate.

1. A gated semiconductor device, comprising: a fin-shaped body having afirst dimension extending in a lateral direction parallel to a majorsurface of a substrate, and second dimension extending in a direction atleast substantially vertical and at least substantially perpendicular tothe major surface, the body having a first side and a second sideopposite the first side; a first gate overlying the first side, thefirst gate having a first gate length in the lateral direction; and asecond gate overlying the second side, the second gate having a secondgate length in the lateral direction which is different from the firstgate length, the second gate being electrically isolated from the firstgate.
 2. The gated semiconductor device as claimed in claim 1, whereinthe first gate consists essentially of polycrystalline silicon germaniumand the second gate consists essentially of polysilicon.
 3. A gatedsemiconductor device, comprising: a fin-shaped body having a firstdimension extending in a lateral direction parallel to a major surfaceof a substrate, and second dimension extending in a direction at leastsubstantially vertical and at least substantially perpendicular to themajor surface, the body having a first side and a second side oppositethe first side; a first gate consisting essentially of polycrystallinesilicon germanium overlying the first side, the first gate having afirst gate length in the lateral direction; and a second gate consistingessentially of polysilicon overlying the second side, the second gatehaving a second gate length in the lateral direction which is differentfrom the first gate length, the second gate being electrically isolatedfrom the first gate.
 4. The gated semiconductor device as claimed inclaim 3, wherein the first gate length is substantially shorter than thesecond gate length such that the first gate has lower gate capacitancerelative to the body than the second gate.
 5. The gated semiconductordevice as claimed in claim 3, further comprising a first gate dielectricdisposed on the first side underlying the first gate, and a second gatedielectric disposed on the second side underlying the second gate. 6.The gated semiconductor device as claimed in claim 3, further comprisinga first oxide region including at least one of an oxide of silicon andan oxide of germanium disposed laterally adjacent to the first gate. 7.The gated semiconductor device as claimed in claim 6, further comprisinga second oxide region including an oxide of silicon disposed laterallyadjacent to the second gate.
 8. The gated semiconductor device asclaimed in claim 7, further comprising a first spacer disposed laterallyadjacent to the first oxide region and a second spacer disposedlaterally adjacent to the second oxide region.
 9. The gatedsemiconductor device as claimed in claim 8, wherein the body is disposedin a semiconductor-on-insulator (SOI) layer of an SOI substrate.
 10. Thegated semiconductor device as claimed in claim 9, wherein the first gateand the second gate contact a buried oxide (BOX) layer of the substrateunderlying the SOI layer.
 11. The gated semiconductor device as claimedin claim 10, wherein the body is electrically insulated from the firstgate and the second gate by the BOX layer, the first gate dielectric,the second gate dielectric and a dielectric cap disposed over a top sideof the body.
 12. The gated semiconductor device as claimed in claim 4,further comprising a source disposed at one end of the body, and a draindisposed at another end of the body opposite the one end, wherein thebody has predominantly a first dopant type selected from n-type andp-type and the source and drain both have predominantly a second dopanttype selected from n-type and p-type, the second dopant type beingdifferent from the first dopant type.
 13. The gated semiconductor deviceas claimed in claim 12, wherein the second gate is conductivelyconnected to one of the source and the drain such that the gatedsemiconductor device is operable as a gated diode.
 14. The gatedsemiconductor device as claimed in claim 12, wherein the gatedsemiconductor device is operable as an insulated gate field effecttransistor (FET), wherein the first gate is operable to apply a bias toadjust a threshold voltage of the FET.
 15. The gated semiconductordevice as claimed in claim 3, further comprising a first gatepolysilicon layer overlying the first gate.
 16. The gated semiconductordevice as claimed in claim 15, further comprising a first silicideregion disposed on the first gate polysilicon layer and a secondsilicide region disposed on the second gate.
 17. The gated semiconductordevice as claimed in claim 16, further comprising a first conductive viacontacting the first silicide region and a second conductive viaconductive via contacting the second silicide region.
 18. An integratedcircuit including the gated semiconductor device as claimed in claim 1.19. A method of making a gated semiconductor device, comprising:patterning a single-crystal semiconductor region of a substrate toextend in a lateral direction parallel to a major surface of a substrateand to extend in a direction at least substantially vertical and atleast substantially perpendicular to the major surface, thesemiconductor region having a first side and a second side opposite thefirst side; forming a first gate overlying the first side, the firstgate having a first gate length in the lateral direction; and forming asecond gate overlying the second side, the second gate having a secondgate length in the lateral direction which is different from the firstgate length, the second gate being electrically isolated from the firstgate.
 20. The method as claimed in claim 19, wherein the first gate isformed to consist essentially of polycrystalline silicon germanium andthe second gate is formed to consist essentially of polysilicon.
 21. Themethod as claimed in claim 20, wherein the first gate and the secondgate are formed by: forming a first region consisting essentially ofpolycrystalline silicon germanium overlying the first side; thereafterforming a second region consisting essentially of polysilicon overlyingthe second side and insulated from the first region; simultaneouslylithographically patterning the first region and the second region usinga given opening of a given lithographic mask, and thereaftersimultaneously thermally oxidizing the first region to form the firstgate and the second region to form the second gate.
 22. The method asclaimed in claim 21, wherein the first region is thermally oxidized at afaster rate than the second region, causing the first gate length to beshorter than the second gate length.